1. Field of the Invention
The present invention relates to isolating DC--DC converters used for switching power supplies, the converters outputting constant voltages.
2. Description of the Related Art
As constant-voltage outputting circuits, isolating DC--DC converters are known. FIG. 11 shows a conventional circuit of a forward-converter type, which is disclosed in Japanese Unexamined Patent Publication No. 3-235668.
In this figure, reference numeral 1 denotes an input terminal, reference numeral 1a denotes (+) input, reference numeral 1b denotes (-) input, reference numeral 2 denotes a main switching element, reference numeral 3 denotes a diode, reference numeral 4 denotes a main transformer, reference numeral 5 denotes a synchronous rectifier (a rectifying-side synchronous rectifier) which is electrically connected when the main switching element 2 is turned on, reference numeral 6 denotes a synchronous rectifier (a commutating-side synchronous rectifier) which is electrically connected when the main switching element 2 is turned off, reference numeral 7 denotes a drive switching element of the commutating-side synchronous rectifier 6, reference numeral 8 denotes a choke coil, reference numeral 9 denotes a capacitor, and reference character D denotes a parasitic diode of each element. In addition, reference numeral 10 denotes an output terminal, reference numeral 10a denotes (+) output, reference numeral 10b denotes (-) output, and the switching elements 2, 5, 6, and 7 are N-channel MOS FETs.
In this circuit, DC power inputted from the input terminal 1 is converted into AC power by the switching operation of the main switching element 2. During an ON-period of the main switching element 2, the AC power is coupled to the secondary-side circuit by the main transformer 4. The AC power is rectified by the synchronous rectifiers 5 and 6 a DC power and filtered by the choke coil 8 and the capacitor 9 to be outputted from the output terminal 10.
The rectifying-side synchronous rectifier 5 and the drive switching element 7 are driven to be turned on when the main switching element is turned on by changes in a voltage of the main transformer 4 occurring due to the switching operation of the main switching element 2. Therefore, during the ON-period of the main switching element 2, on the secondary side of the main transformer 4, current flows in a path indicated by the solid line shown in the figure, and electromagnetic energy is charged in the choke coil 8, by which an output voltage is provided at the output terminal 10.
When the main switching element 2 is turned off and a reset pulse is generated in the main transformer 4, the input capacitance of the commutating-side synchronous rectifier 6 is charged via a parasitic diode D of the drive switching element 7, by which the commutating-side synchronous rectifier 6 is turned on. Even if the reset of the main transformer 4 is completed in the course of the OFF-period of the main switching element 2 and the reset pulse disappears, the drive switching element 7 maintains the OFF-period until the main switching element 2 is turned on. Thus, since the path for releasing the charge of the input capacitance of the synchronous rectifier 6 is closed during the OFF-period of the drive switching element 7, the commutating-side synchronous rectifier 6 maintains the ON-state. During the OFF-period of the main switching element 2, since current flows over a path indicated by the dotted line shown in the figure by electromagnetic energy charged in the choke coil 8, an output voltage is outputted from the output terminal 10.
In the above conventional circuit, the ON/OFF operation of the rectifying-side synchronous rectifier 5 is synchronized to the ON/OFF operation of the main switching element 2. In addition, since the commutating-side synchronous rectifier 6 can be turned on over the entire OFF-period of the main switching element 2 by providing the drive switching element 7, circuit efficiency can be improved.
In the conventional circuit, the rectifying-side synchronous rectifier 5 and the drive switching element 7 can be turned on by synchronizing to the turn-on of the main switching element 2. However, after the turn-on of the drive switching element 7, charge of the input capacitance of the commutating-side synchronous rectifier 6 passes through the drive switching element 7 to be discharged, by which the commutating-side synchronous rectifier 6 is turned off. As a result, there is a time lag of a few tens of nsc until the commutating-side synchronous rectifier 6 is turned off after the turn-on of the drive switching element 7. With this time lag, since the commutating-side synchronous rectifier 6 remains in an ON-state shortly after the main switching element 2 is turned on and the rectifying-side synchronous rectifier 5 is turned on, the secondary coil of the main transformer 4 is short-circuited. With the short-circuit phenomenon, a large amount of short-circuit current flows over a path sequentially passing through the secondary coil of the main transformer 4, the commutating-side synchronous rectifier 6, the rectifying-side synchronous rectifier 5, and the secondary coil of the main transformer 4.
The short-circuit current flows during the OFF-period of the commutating-side synchronous rectifier 6 after the turn-on of the rectifying-side synchronous rectifier 5. The flow of the short-circuit current produces large conductive losses and noises which are not negligible. Thus, primarily, although improvement in circuit efficiency is intended to be achieved by synchronizing the operations of the rectifiers 5 and 6 to the ON/OFF operation of the main switching element 2, the conductive losses due to the short-circuit current deteriorates the circuit efficiency, and the occurrence of noises worsens the performance of circuit operations.
Particularly, since the conductive losses due to the short-circuit current increases in proportion to the switching frequency of the main switching element 2, the conventional circuit has a disadvantage in terms of obtaining a higher switching frequency in order to reduce the size of a DC--DC converter.